P of F Question [Archive]

antic81

2nd Apr 2007, 12:38

bri1980

2nd Apr 2007, 18:31

Effect on stability is positive as far as I know. Has the same effect as dihedral.

There is a good article at

http://en.wikipedia.org/wiki/Flight_dynamics

the maths is quite heavy-going though! :)

Go Smoke

2nd Apr 2007, 18:42

Hi there
A friend of mine has just done the Principals of Flight paper at Gatwick, one of his questions was:
What effect does sweep back have on directional Stability?
A. Positive
B. Negative
C. None
I cant seem to settle on an answer, probably because I have not reached it yet in my studdies!
Hi There, it has a positive effect on directional stability.
If you try and imagine the aircraft yawing/slipping to the right for instance then then more of the left wing (greater area and linear length) is presented to the airflow and thus more lift is created on that wing. With more lift comes more drag and, as a result that wing reduces speed and the aircraft yaws back towards it's original heading.
Can't think of a better way to explain it - hope it makes some sense.

High Wing Drifter

3rd Apr 2007, 08:10

An interesting question. Thinking back to GS, sweep increases roll stability, but I'm not sure about directional.

If "directional" means the heading or the direction the nose is pointing then I would have thought that it decreases stability, because sweep tends to be more predesposed to dutch roll tendancies unless some other fix as applied such as really large fins, reduced dihedral, yaw dampers, etc.

If "directional" means track than I would have thought C is the answer.

I suspect "directional" references the heading. My thoughts would be that with all other things considered constant, increased sweep reduces directional stability. In which case, I would have thought B ("Negative") is the answer.

Jaguar Pilot

3rd Apr 2007, 08:41

Go Smoke is on the right track.

Uncommanded yaw results in an effective change in aspect ratio due to sweepback. Drag increases on the wing which is opposite to the direction of yaw. Thus sweepback (described in JAA questions as positive sweep) contributes to directional stability as well as lateral.

High Wing Drifter

3rd Apr 2007, 19:32

Jag,

I agree yes, that the nose will attempt to return to the original position, but the restoring force is theorectically excessive and the question isn't specific as to whether we are talking about static or dynamic stability. One of the key characteristics of swept wing aircraft is the dutch roll, therefore one of the key defining characteristics of swept wing aircraft is a comparatively weak directional stability when compared to spiral stability leading to much more neutral and possibly unstable dynamic directional stability! Hence yaw damper systems.

I have no idea what answer they are looking for, but in my mind adding wing sweep to the equation prioritises reduced directional stability considerations.

Jaguar Pilot

4th Apr 2007, 09:58

High Wing Drifter

Let me assure you that the exam answer is positive effect,
but I am not at liberty to tell you how I know.;)

JP

Adj

4th Apr 2007, 21:56

Maybe you answered this question in your Jaa's as Positive and got 100% in PoF?!?
That would be a good explanation of how you would know for sure ;)

adj

antic81

6th Apr 2007, 08:34

Thanks for the responses guys!

its good news for my mate because he told me his answer was positive.

Thanks again for your insight on this one!

I feel certain I will return with more in the near future!

Cheers

Anthony

Superpilot

6th Apr 2007, 20:36

That's what I put antic81. Say, you aren't that Dom Jolly Look-alike who finished PoF in about 13 minutes and Perf in about 9 are you?

antic81

8th Apr 2007, 14:02

Superpilot,

I wish!
(not so much about looking like Dom Jolly)
I am still studying hard and have yet to write!
A mate of mine was doing his first set of seven last week, I am still far from ready!(He had a slight advantage as he already holds an ATPL, just not a JAR one.)

Cheers

Ant